1. Field of the Invention
The present invention relates to a miter saw which includes a miter saw unit including a rotary tool such as a circular saw blade and drive means for driving the rotary tool and adapted to be vertically pivoted relative to the upper surface of a base on which a workpiece is placed so as to carry out cutting or similar operations of the workpiece on the base.
2. Description of the Prior Art
Conventional miter saws generally include a base having a base body on which a workpiece is placed and a fixed arm vertically mounted on the base body, and a miter saw unit including a rotary tool such as a circular saw blade and drive means for driving the rotary tool and connected to the fixed arm for vertical swinging movement relative to the base. Biasing means is provided between the fixed arm and the miter saw unit so as to normally urge the miter saw unit for upward pivotal movement. The biasing means may typically include a torsion spring encircling in coils the pivotal shaft between the fixed arm and the miter saw unit or a tension spring positioned between the fixed arm and the miter saw unit.
In such prior art saws with the biasing means using a torsion spring, however, loss in general movement of the spring occurs due to coil-to-coil friction in coils of the spring, contact of the spring with the support shaft (pivotal shaft encircled by the spring), or contact of end arms of the spring with external portions (portions of the fixed arm or the miter saw unit). Further, dimensional tolerances of the spring itself in manufacturing is large, increasing the tendency of variations in load at initial assembling. In order to counteract this tendency, the biasing force must be set relatively high.
Such relatively high setting cf the biasing force may cause a heavier pivotal movement of the miter saw unit in the vertical direction or abrupt upward swinging return movement thereof, preventing smooth pivotal movement of the miter saw unit.
Furthermore, in case of such a prior art saw employing a torsion spring, some retaining techniques for the torsion spring will cause a tendency of stress concentration or increase of moment adjacent the ends or arms of the torsion spring, so that fatigue of the torsion spring must be taken into consideration.
In case of biasing means employing a tension spring, there is a tendency of variation in its initial tension at manufacturing and loss of movement will occur due to contact of hooked portions at both ends of the tension spring with external portions (engaging portions of the fixed arm or the miter saw unit to be engaged with the hooked portions), and consequently, the biasing force must be set relatively high at assembling, as with the case of the torsion spring. This also constitutes a significant obstacle to smooth pivotal movement of the miter saw unit.
Furthermore, in case a tension spring is employed, there is disadvantageously a tendency of stress concentration on the hooked portions at both ends of the tension spring which may cause breakage thereof. In order to avoid such breakage, the spring itself must be of a substantially large size, which will also prevent smooth pivotal movement of the miter saw unit.
In either case of the above prior art springs, the springs further have the disadvantage of being costly to manufacture.